This disclosure generally relates to systems and methods for balancing rotating machinery to reduce or minimize vibrations. In particular, the disclosed embodiments relate to systems and methods for balancing gas turbine engines.
It is either impossible or nearly impossible, as a practical matter, to build a rotating structure that is perfectly balanced upon manufacture. Any such structure will produce a certain amount of undesired vibration to a greater or lesser extent. Such vibration is usually passed through mounts that restrain the rotating part of the structure, and can therefore manifest itself as unwanted noise or vibration in adjacent structures. As is known to those skilled in the art, synchronous vibration may be characterized by an amplitude (i.e., magnitude) and a phase angle (i.e., direction). Thus, the vibration of a part may be represented as a vector or phasor.
One type of rotating machinery susceptible to undesired vibration is the high-bypass turbofan engine used in commercial aviation. Such engines have a large number of rotating elements. These rotating elements can be grouped according to the relative speed of rotation. Some of the rotating elements form a low-speed rotating system and other rotating elements form one or more high-speed rotating systems. More specifically, each rotating system of a gas turbine engine comprises an upstream rotating multi-stage compressor connected to a downstream multi-stage turbine by means of a shaft. The low-pressure turbine and low-pressure compressor are connected by a low-pressure shaft; the high-pressure turbine and high-pressure compressor are connected by a high-pressure shaft which surrounds a portion of the low-pressure shaft, with the high-pressure compressor and turbine being disposed between the low-pressure compressor and turbine. The fan of the turbofan engine is the first stage of the low-pressure compressor. Vibration caused by unbalances in the various stages of a turbofan engine contributes to wear and fatigue in engine components and surrounding structures, and unwanted noise in the passenger cabin of the airplane.
One way of reducing structurally transmitted vibrations is to balance the rotating systems of aircraft engines on a regular basis. Engine balancing is well known in the aircraft art. The manufacturers of turbofan engines have developed techniques for controlling the magnitude of unwanted vibration by affixing balancing mass to the engine. Typically, only the fan and the last stage of the low-pressure turbine of a turbofan engine are accessible for applying balancing mass after the engine is manufactured or assembled. Internal stages are inaccessible as a practical matter.
A known method for applying balancing mass involves the selection of a combination of balancing screws from a set of screws of different standard mass, with screws being threadably inserted into respective threaded holes located around an outer periphery of an internal turbofan engine component (such as a fan spinner). For example, to achieve a balance, one or more screws of the same mass or different masses can be screwed into respective threaded holes, thereby producing a center of gravity which is closer to the axis of rotation than was the case without balancing. The total effect of multiple attached balancing masses can be determined by treating each mass and its respective location as a vector, originating at the axis of rotation, and performing a vector sum.
Although the unbalances of accessible stages of a turbofan engine are the primary contributors to engine vibration, the unbalances that often reside at inaccessible engine stages also contribute to overall engine vibration. When corrective masses can only be placed on the two accessible stages, it is difficult to select masses of the proper magnitude and angular position such that they not only function to reduce vibration caused by specific unbalances at the fan and last stage of the low-pressure turbine, but also reduce the influence of unbalances at the remaining/other stages of the low-pressure compressor and turbine, as well as over the operating envelope of the engine.
The specification of the location and amount of mass to be applied to a rotating system in order to balance it is referred to herein as the balance solution for the rotating system. In order to determine balance solutions for rotating systems of turbofan engines, vibration data is obtained. Vibration data is a measure of the amount of vibration that an engine is producing at various locations as the engine is operated at various speeds and through ranges of other parameters. Vibration data can be gathered at an engine balancing facility located on the ground or during flight. If accelerometers are used to capture rotating system vibration response, synchronous vibration data may be derived using a keyphasor index on the rotating system. While multiple methods known to the art can be used to capture and derive vibration data, that data must contain a displacement as well as a phase corresponding to synchronous vibration. After vibration data is obtained, the vibration data is used to derive a balance solution that attempts to minimize the vibration of the engine producing the data.
In a known procedure for gathering engine vibration data in flight, so-called “stable” vibration data is captured using the last stable point for each of six speed ranges. Sufficient stability is established by monitoring amplitude over a period of time and verifying that the amplitude variation is within an acceptable predefined limit. (Stability can also be determined from phase and N1 speed remaining within a given range for a given time.) The corresponding shaft speed is also captured by recording outputs from a tachometer or other shaft speed sensor. A respective “stable” vibration data point is captured during the flight (vibration amplitude and phase) for each range of shaft speed. If stability is achieved, the new vibration data point is substituted for the present “stable” vibration data point in the corresponding shaft speed range. After the aircraft has landed, the “stable” vibration data points (vibration amplitude and phase) recorded during the flight for the six shaft speed ranges are extracted and then a least squares amplitude solution is calculated using the six speed range points and general in-flight influence coefficients. (The method of least squares is a standard approach to the approximate solution of overdetermined systems, i.e., sets of equations in which there are more equations than unknowns. “Least squares” means that the overall solution minimizes the sum of the squares of the errors made in the results of every single equation.) The output of this analysis process is adopted as the proposed balance solution. This method intrinsically assumes that there is a unique relationship between engine speed and vibration response; for example, a particular engine speed always produces a similar vibration response.
Dynamic unbalance characteristics cause sub-optimal balance solutions using state-of-the-art balancing techniques. Existing techniques do not account for dynamic characteristics related to parameters other than shaft speed, resulting in solutions that require multiple attempts to obtain acceptable balance states. There is a need for improved methods of balancing turbofan engines having dynamic unbalance characteristics to minimize vibrations.